Communication apparatus and method

ABSTRACT

A communication apparatus includes a receiving device configured to receive a physical frame in which a plurality of MAC frames, each having a sequence number, are aggregated. A reception buffer which stores MAC frames of the physical frame received by the receiving device. A determination device determines whether or not the first MAC frame in the physical frame received by the receiving device has been successfully received. A reception buffer management device determines, when the first MAC frame has been successfully received, a MAC frame to be forwarded to an upper layer, on the basis of the sequence number of the first MAC frame, and extracts the MAC frame from the reception buffer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromprior Japanese Patent Application No. 2004-304475, filed Oct. 19, 2004,the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a communication apparatus and methodwhich perform Media Access Control on the basis of the carrier senseinformation of a physical layer and the carrier sense information of aMAC layer.

2. Description of the Related Art

The Media Access Control (MAC) is control for causing a plurality ofcommunication apparatuses which perform communication while sharing thesame wireless media to decide how to use the wireless media intransmitting communication data or management frame. Owing to MediaAccess Control, even if two or more communication apparatuses transmitcommunication data or management frame by using the same wireless mediaat the same time, there is less chance of the occurrence of a phenomenon(collision) in which a communication apparatus on the receiving sidecannot decode communication data properly. Media Access Control is alsoa technique for controlling access from communication apparatuses to amedia so as to minimize the chance of the occurrence of a phenomenon inwhich, despite the presence of communication apparatuses havingtransmission requests, the media is not used by any of the communicationapparatuses.

However, especially in wireless communication, it is difficult tosimultaneously monitor transmission data (or management) frame while thecommunication apparatus transmits the frame, and therefore the MediaAccess Control (MAC) is required in which collision detection is notassumed. A typical technique standard of wireless LAN IEEE802.11 adoptsCSMA/CA (Carrier Sense Multiple Access with Collision Avoidance). TheCSMA/CA is designed to reduce the collision probability. In IEEE802.11,the MAC header has the Duration value which is the time, inmicroseconds, required to transmit the data or management frame(including the time of SIFS interval). In this period, a communicationapparatus which is irrelevant to the frame exchange sequence and has notransmission right waits for transmission upon determining a virtualbusy state of the media. This prevents the occurrence of collision. IEEE802.11 defines that the state of a wireless media is determined on thebasis of such a combination of virtual carrier-sense on a MAC layer andphysical carrier sense on a physical (PHY) layer, and Media AccessControl is performed on the basis of the determination.

IEEE 802.11 using CSMA/CA has increased the communication speed mainlyby changing the physical layer protocol. With regard to the 2.4 GHzband, there have been changes from IEEE 802.11 (established in 1997, 2Mbps) to IEEE 802.11b (established in 1999, 11 Mbps), and further toIEEE 802.11g (established in 2003, 54 Mbps). With regard to the 5 GHZband, only IEEE 802.11a (established in 1999, 54 Mbps) exists as astandard. In order to develop standard specifications directed tofurther increase communication speeds in both the 2.4 GHz band and the 5GHz band, IEEE 802.11 TGn (Task Group n) has already been established.

In addition, several access control techniques designed to improve QoS(Quality of Service) are known. For example, as a QoS technique ofguaranteeing parameters such as a designated bandwidth and delay time,HCCA (HCF Controlled Channel Access) which is an extended scheme of aconventional polling sequence is available. According to HCCA,scheduling is performed in a polling sequence in consideration ofrequired quality so as to guarantee parameters such as a bandwidth anddelay time. Jpn. Pat. Appln. KOKAI Publication No. 2002-314546 refers toQoS in the IEEE 802.11e standard, and discloses a method of assigningpriorities to communications between communication apparatuses in awireless network.

When the same frequency band as that in the existing specifications isto be used in realizing an increase in communication speed, it ispreferable to assure coexistence with communication apparatusesconforming to the existing specifications and to maintain backwardcompatibility. For this reason, it is basically preferable that aprotocol on a MAC layer conforms to CSMA/CA matching the existingspecifications. In this case, a temporal parameter associated withCSMA/CA, e.g., an IFS (InterFrame Space) or random backoff period needsto match that in the existing specifications.

Even if an attempt to increase the communication speed in terms ofphysical layer succeeds, the effective throughput of communicationcannot be improved. That is, when an increase in the communication speedof the physical layer is realized, the format of a PHY frame (PHYpreamble and PHY header) ceases to be effective any more. An increase inoverhead due to this may hinder an increase in throughput. In a PHYframe, a temporal parameter associated with CSMA/CA is permanentlyattached to a MAC frame. In addition, a PHY frame header is required foreach MAC frame.

As a method of reducing overhead and increasing throughput, a Block Acktechnique introduced in recently drafted IEEE 802.11e/draft 5.0(enhancement of QoS in IEEE 802.11) is available. The Block Acktechnique can consecutively transmit a plurality of MAC frames withoutany random backoff, and hence can reduce the backoff amount to somedegree. However, a physical layer header cannot be effectively reduced.In addition, according to aggregation introduced in initially draftedIEEE 802.11e, both the backoff amount and the physical layer header canbe reduced. However, since the length of a physical layer framecontaining MAC frames cannot be increased beyond about 4 kbytes underthe conventional limitation on the physical layer, an improvement inefficiency is greatly limited. Even if the length of a PHY layer framecan be increased, another problem arises, i.e., a reduction in errortolerance.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to a communication apparatus andmethod which can coexist with an existing apparatus and can improve thesubstantial communication throughput by eliminating overheadaccompanying the transmission of a plurality of frames by making a frameformat more efficient.

According to an aspect of the present invention, there is provided acommunication apparatus comprising a receiving device configured toreceive a physical frame in which a plurality of MAC frames, each havinga sequence number, are aggregated, a reception buffer which stores MACframes of the physical frame received by the receiving device, adetermination device configured to determine whether or not a first MACframe in the physical frame received by the receiving device has beensuccessfully received, and a reception buffer management deviceconfigured to, when the first MAC frame has been successfully received,determine a MAC frame to be forwarded to an upper layer, on the basis ofa sequence number of the first MAC frame, and extract the MAC frame fromthe reception buffer so as to forward the MAC frame to the upper layer.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a block diagram showing the arrangement of a communicationapparatus according to an embodiment of the present invention;

FIG. 2 is a view showing an example of an immediate Block Ack sequencein HCCA (Hybrid coordination function Controlled Channel Access);

FIG. 3 is a view for explaining buffer management to be performed on thereceiving side when data transmission is performed by using the BlockAck defined in IEEE 802.11e;

FIG. 4 is a view showing the format of a physical frame in which aplurality of MPDUs are aggregated into one PSDU;

FIG. 5 is a view showing another example of the format of a physicalframe in which a plurality of MPDUs are aggregated into one PSDU;

FIG. 6 is a view showing the format of a Compressed Block Ack;

FIG. 7 is a view showing an example of the transmission of a physicalframe which has a plurality of MPDUs aggregated into a single PSDU andcontains no Block Ack Request;

FIG. 8 is a view for explaining reception buffer management according tothe first embodiment of the present invention;

FIG. 9 is a view showing another example of reception buffer management;

FIG. 10 is a view showing still another example of reception buffermanagement;

FIG. 11 is a view for explaining a buffer management technique to beexecuted in consideration of the window size on the receiving side;

FIG. 12 is a view for explaining an example of how the reception bufferis cleared according to the second embodiment of the present invention;

FIG. 13 is a view for explaining an example of how the reception bufferis cleared according to the second embodiment of the present invention;

FIG. 14 is a view for explaining another example of how the receptionbuffer is cleared according to the second embodiment of the presentinvention;

FIG. 15 is a view for explaining still another example of how thereception buffer is cleared according to the second embodiment of thepresent invention;

FIG. 16 is a view for explaining still another example of how thereception buffer is cleared according to the second embodiment of thepresent invention;

FIG. 17 is a view showing an example of the creation of a CompressedBlock Ack Bitmap;

FIG. 18 is a view showing a retransmission sequence based on aCompressed Block Ack Bitmap;

FIG. 19 is a view showing errors in retransmitted frames;

FIG. 20 is a view showing an example of Block Ack Control according tothe third embodiment of the present invention;

FIGS. 21A and 21B are views showing a Block Ack Control field accordingto the third embodiment of the present invention;

FIG. 22 is a view showing another example of Block Ack control accordingto the third embodiment of the present invention;

FIG. 23 is a view showing a case wherein a Compressed Block Ack Bitmapis created in correspondence with sequence number numbers;

FIG. 24 is a view showing a retransmission sequence based on theCompressed Block Ack Bitmap created in correspondence with the sequencenumber positions; and

FIG. 25 is a block diagram showing the detailed arrangement of acommunication apparatus according to an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a block diagram showing the arrangement of a communicationapparatus according to the first embodiment of the present invention. Acommunication apparatus 100 is an apparatus configured to communicatewith another communication apparatus through a wireless link, andincludes processing units 101, 102, and 103 respectively correspondingto a physical (PHY) layer, MAC layer, and link layer. These processingunits are implemented as analog or digital electronic circuits inaccordance with implementation requirements. Alternatively, theprocessing units are implemented as firmware or the like to be executedby a CPU incorporated in an LSI. An antenna 104 is connected to the PHYlayer processing unit (“processing unit” will be omitted hereinafter)101. The MAC layer 102 includes an aggregation processing device 105 forMAC frames. The aggregation processing device 105 includes a carriersense control device 106 and retransmission control device 107, andperforms transmission/reception of Block Ack (partial acknowledgementframe for a plurality of MAC frames) frames (to be described in detaillater), retransmission control based on Block Ack frames, and the like.

The physical layer 101 is designed to be compatible with two types ofphysical layer protocols. The processing unit 101 includes a first-typephysical layer protocol processing device 109 and a second-type physicallayer protocol processing device 110 for the respective types ofprotocol processing. The first-type physical layer protocol processingdevice 109 and second-type physical layer protocol processing device 110often share circuits and are not necessarily independent of each otherin terms of implementation.

In this embodiment of the present invention, the first-type physicallayer protocol is assumed to be a protocol defined in IEEE 802.11a, andthe second-type physical layer protocol is assumed to be a protocolusing a so-called MIMO (Multiple Input Multiple Output) technique usinga plurality antennas on each of the transmitting side and the receivingside. Using the MIMO technique makes it possible to expect an increasein transmission capacity almost proportional to the number of antennaswithout changing the frequency band. The MIMO technique is therefore atechnique directed to further increase the throughput of IEEE 802.11.Note that the link layer 103 has a normal link layer function defined inIEEE 802.11. The technique to be used to increase the transmission rateis not limited to MIMO. For example, a method of increasing the occupiedfrequency band may be used or may be combined with MIMO.

FIG. 25 is a block diagram showing the detailed arrangement of acommunication apparatus according to the first embodiment of the presentinvention. The MAC layer 102 includes an aggregation processing device105 for MAC frames and a reception buffer 113. The aggregationprocessing device 105 includes a carrier sense control device 106,retransmission control device 107, reception determination processingdevice 111, and reception buffer control device 112. The receptionbuffer 113 includes a buffer region wherein MAC frames that are normallyreceived are stored. The carrier sense control device 106 recognizesthat the wireless medium is in a busy state, while the physical frame inwhich multiple MAC frames are aggregated, is received through thephysical layer 101. On the contrary, the carrier sense control device106 recognizes that the wireless medium is in an idle state, if thewireless medium is available (not used). To be described later, all ofthe aggregated MAC frames are checked by the reception determinationprocessing device 111 whether each of them has been successfullyreceived or not. The retransmission control device 107 notifies thereception status of each of the MAC frames, to the transmitting side bytransmitting a Block Ack frame, which includes a multipleacknowledgements for the multiple MAC frames. The retransmission controldevice 107 performs retransmission control based on Block Ack Request(request for partial acknowledgement) frames and Block Ack framesresponding to them. The MAC frame which has been successfully receivedis stored in the reception buffer 113. The determination whether the MACframe has been successfully received or not is based on a result of a(CRC) check performed by the reception determination processing device111. In the case of a Block Ack Request frame, information thereof isnotified to the retransmission control device 107.

In the first embodiment of the present invention, MAC frames areextracted from the reception buffer 113 based on the determinationresult of the reception determination processing device 111, andforwarded to the upper layer (link layer 103).

According to IEEE 802.11e/Draft 8.0, as a technique of improving thetransmission efficiency at the MAC (Media Access Control) layer, a BlockAck technique has been proposed. In the Block Ack technique, a giventerminal transmits QoS (Quality of Service) data at minimum frameintervals called SIFS (Short InterFrame Space) for a given channel useperiod (TXOP: Transmission Opportunity). Thereafter, the terminaltransmits a Block Ack Request frame to the receiving terminal at anarbitrary timing to request its reception status. The receiving sideconverts the reception status into information in the bitmap format onthe basis of the starting sequence number (Block Ack Starting SequenceControl) determined by the Block Ack Request, and returns theinformation as a Block Ack.

Prior to the detailed description of the embodiments of the presentinvention, existing techniques for Block Acks and buffer management on adata receiving terminal for Block acknowledgement will be described.According to IEEE 802.11e/Draft 10.0, as a technique of improving thetransmission efficiency at the MAC (Media Access Control) layer, a Blockacknowledgement technique is known. In the Block acknowledgementtechnique, a given transmitting terminal transmits QoS (Quality ofService) data at minimum frame intervals called SIFS (Short InterFrameSpace) for a given channel use period (TXOP: Transmission Opportunity).Thereafter, the transmitting terminal transmits a Block Ack Request tothe receiving terminal to request its reception status (partialacknowledgement) at an arbitrary timing. The receiving side converts thereception status into information in the bitmap format on the basis ofthe starting sequence number (Block Ack Starting Sequence Control)determined by the Block Ack Request, and returns the information as aBlock Ack.

FIG. 2 shows an example of an immediate Block Ack sequence in HCCA(Hybrid coordinate function Controlled Channel Access). HCCA is acontention-free QoS control scheme, in which a QOS-AP (QoS Access Point;it can be called “QAP”) called an HC (Hybrid Coordinator) serves as anentity which performs bandwidth management including the allocation ofTXOPs to QSTAs (QoS stations). The HC shown in FIG. 2 transmits a QoSCF-Poll (Contention Free Poll) frame 20 to a QSTA (QoS Station) 1 andgives it a permission to use a channel as TXOP period 1. The QSTA 1transmits QoS data 21 for TXOP period 1 at SIFS intervals in a burstmanner, and then transmits a Block Ack Request 22. The Block Ack Request22 includes a 2-octet field called Block Ack Starting Sequence Control,in which a fragment number and starting sequence number (Starting Seq inthe FIG. 2) are written. A QSTA 2 on the data receiving side generates aBlock Ack 23 by converting a reception status starting with the fragmentnumber and starting sequence number into information in the bitmapformat (Block Ack Bitmap), and notifies the QSTA 1 on the datatransmitting side of the Block Ack 23.

More specifically, as shown in FIG. 2, the QSTA 1 transmits first theQoS data 21 with sequence numbers “1”, “2”, and “3” to the QSTA 2 atSIFS intervals in a burst manner, and then transmits the Block AckRequest 22 upon setting the Block Ack Starting Sequence Control value to“1”. The QSTA 2 receives the Block Ack Request 22. Since the Block AckStarting Sequence Control value of the Block Ack Request 22 is “1”, theQSTA 2 writes a relative reception status from sequence number “1” in aBlock Ack Bitmap, and transmits the Block Ack 23 to the QSTA 1.

In the example shown in FIG. 2, the result of FCS (Frame Check Sequence)calculation indicates that an error has occurred in the data withsequence number “1”. Although the Block Ack Starting Sequence Controlvalue of the Block Ack 23 is “1”, the Block Ack Bitmap indicates thatonly the data with sequence numbers “2” and “3” have been successfullyreceived, by using the bits in the corresponding positions. In general,the Block acknowledgement technique can notify a reception statuscorresponding to a maximum of 64 MSDUs (MAC Service Data Units),including a case of fragmentation. The MSDU is information that isdelivered as a unit between MAC-SAPS (service access points). The MPDUis the unit of data exchanged between two peer MAC entities using theservices of the PHY layer. According to the IEEE 802.11 standard, oneMSDU is fragmented into a maximum of 16 MPDUs (MAC Protocol Data Units).Therefore, a Block Ack Bitmap field contained in a Block Ack frame has asize of 1,024 (64 MSDUs×16) bits. According to the IEEE 802.11 standard,a Retry Limit and Lifetime are set for a data frame to be transmitted.Any frame that exceeds the Retry Limit or Lifetime is discarded on thetransmitting side. According to the IEEE 802.11e standard, a Delay Boundis associated with each frame in accordance with trafficcharacteristics. The Delay Bound contains an unsigned integer thatspecifies the maximum amount of time, in microseconds, allowed totransport an MSDU belonging to the TS (Traffic Stream) in this TSPEC(Traffic Specification), measured between the time marking the arrivalof the MSDU at the local MAC sublayer from the local MAC-SAP and thetime of completion of the successful transmission or retransmission ofthe MSDU to the destination. For example, for traffic required to meet areal-time transmission like voice data, a relatively short Delay Boundis set. When the Delay Bound is exceeded, the corresponding frame isalso discarded on the transmitting side. In the example shown in FIG. 2,the QoS data frame with sequence number “1” needs to be retransmitted,but the transmitting side has abandoned to retransmit it because theRetry Limit or the like is exceeded.

When the HC assigns the QSTA 1 a TXOP period 2 again to permit the QSTA1 to use the channel by transmitting a QoS CF-Poll frame 24 to the QSTA1 after a lapse of a predetermined amount of time since the expirationof the TXOP period 1, the QSTA 1, which has aborted retransmission ofthe frame with sequence number “1”, is now transmitting data 25 withsequence number “4”. The QSTA 1 then transmits a Block Ack Request 26upon setting the Block Ack Starting Sequence Control value to “4”. TheQSTA 2 returns a Block Ack 27 indicating that “4” has been successfullyreceived. Note that in the example shown in FIG. 2, all the frames havethe same priority. According to the IEEE 802.11e standard, data framesto be transmitted are classified according to the traffic types, and areassigned identifiers called TIDs (Traffic Identifiers). Since a sequencenumber is also assigned for each TID, a Block Ack Request or Block Ackis required for each TID.

FIG. 3 shows how buffer management is performed on the receiving side onthe basis of the contents shown in FIG. 2 when data transmission isperformed by the Block Ack technique defined in IEEE 802.11e. Assumethat a buffer 30 on the receiving side is empty. Assume that thetransmitting terminal transmits data with sequence numbers “1”, “2”, and“3” in a burst manner, and only the data with sequence numbers “2” and“3” have been successfully received, as shown in FIG. 3. On thereceiving side, since the frame with sequence number “1” has not beenreceived, the frames with sequence numbers “2” and “3” are kept storedin the reception buffer 30 at the MAC layer.

In this case, when the frame with sequence number “1” reaches the RetryLimit (or Lifetime, Delay Bound), the transmitting terminal newlytransmits a frame with sequence number “4”, and then transmits a BlockAck Request with the Block Ack Starting Sequence Control value being setto “4”. At this time, the frames with sequence numbers “2”, “3”, and “4”are stored in the buffer 30 on the receiving side. Note that when theBlock Ack Request is received, the Block Ack Starting Sequence Controlvalue is set to “4”. According to the IEEE 802.11e/Draft 10.0 standard,all complete MSDUs with lower sequence numbers than the startingsequence number contained in the Block Ack Request shall be indicated tothe MAC client. Therefore, all the data frames with lower sequencenumbers than the Block Ack Starting Sequence Control value “4”, i.e.,sequence numbers “2” and “3”, are released from the buffer 30 andforwarded to the upper layer. In addition, the frame with sequencenumber “4” consecutive to sequence numbers “2” and “3” is released fromthe buffer 30 and forwarded to the upper layer. The above buffermanagement on the receiving side is the same as that in EDCA (EnhancedDistributed Channel Access) which is a contention-based QoS controlscheme.

First Embodiment

The first embodiment of the present invention is directed to acommunication apparatus designed to aggregate a plurality of MPDUs (MACProtocol Data Units) into a single PSDU (PHY Service Data Unit) andtransmit a PPDU (PHY Protocol Data Unit) containing no Block AckRequest, in which when the first MPDU has been successfully received,reception buffer management is performed on the basis of the sequencenumber of the first MPDU. Note that a PPDU corresponds to a physicalframe containing a PHY Preamble and a PHY Header and PSDU (includingmultiple MPDUs), and an MPDU corresponds to a MAC frame containing a MACheader and MSDU (MAC Service Data Unit).

In order to achieve a high throughput in wireless LAN, the overhead ofthe MAC layer and the PHY layer, such as an interframe space, randombackoff period and PHY header must be reduced. As shown in FIGS. 4 and5, these overheads can be reduced by transmitting a plurality of MPDUsupon aggregating them into one PSDU. In the example shown in FIG. 4, aheader 41 which indicates in octets the length of each MPDU containing aMAC header to an FCS exists in the head of a PSDU 40 in which aplurality of MPDUs are aggregated. The header 41 will be referred to asa MAC super frame header hereinafter. A CRC (Cyclic Redundancy Check)for detecting an error in the header 41 itself is added to the MAC superframe header 41. “0” is written in an MPDU length field corresponding toa portion in which no MPDU exists. In addition, if the MAC super frameheader 41 is incorrect, the reception of all the MPDUs is considered tobe wrong. In the example shown in FIG. 5, in the front portion of eachof the aggregated MPDUs, information indicating the length of acorresponding MPDU exists. In addition, a CRC for detecting an error inthe MPDU length information is added to it. A combination of MPDUinformation and a CRC will be referred to as an MPDU separation 50. Uponreceiving a physical frame (PPDU) having the arrangement shown in FIG.5, a terminal checks the CRC of the MPDU separation 50. If the firstMPDU separation 50 has been successfully received, the terminal extractssucceeding MPDU1 and calculates an FCS. If the FCS calculation result iscorrect, it is determined that MPDU1 has been successfully received. Ifthe FCS calculation result is incorrect, the reception of MPDU1 isconsidered to be wrong. The terminal then checks the CRC of a next MPDUseparation 51 upon skipping a portion indicated by the MPDU lengthinformation. If the MPDU separation 51 is incorrect, the terminalconsecutively performs a CRC check on an octet basis. If the result iscorrect, the FCS for the succeeding MPDU is calculated to determinewhether or not the MPDU has been successfully received.

In the following description concerning the embodiment of the presentinvention, for a partial acknowledgement to a physical frame (PPDU) inwhich a plurality of MPDUs are aggregated in a PSDU, for example, anextended one of the Block Ack defined in IEEE 802.11e is used. FIG. 6shows the frame arrangement of an extended Block Ack. According to IEEE802.11e/Draft 10.0, a Block Ack frame has a bitmap (Block Ack Bitmap)having a fixed length of 1,024 bits in consideration of fragmentation.Since the overhead of a fragment is generally large, in order to achievea high throughput, it is preferable not to fragment an MSDU. An extendedBlock Ack frame therefore includes a Block Ack Bitmap 60 correspondingat most to 64 MSDUs on the premise that no fragmentation is performed,as shown in FIG. 6. This makes it possible to reduce the bitmap size,that is the reception status bitmap, to 1/16 that of a conventionalBlock Ack frame. A Block Ack frame with a compressed Block Ack Bitmapwill be referred to as a “Compressed Block Ack” hereinafter. Note thatthe Block Ack Bitmap of a Compressed Block Ack may have a variablelength in accordance with the number of MPDUs aggregated into one PSDU.

A case wherein a physical frame in which a plurality of MPDUs areaggregated in a signal PSDU is to be transmitted will be described withreference to FIGS. 7 and 8. Consider a case wherein MPDUs with sequencenumbers “1” to “8” are aggregated by a transmitting terminal, as shownin FIG. 7. According to the IEEE 802.11e/Draft 10.0 standard, a BlockAck Request is required to return a Block Ack. The first embodiment is,however, based on the premise that when a physical frame in which aplurality of MPDUs are aggregated is received, a Compressed Block Ack isimmediately returned even if no Block Ack Request is contained in theframe. A Compressed Block Ack corresponds to an acknowledgement obtainedby simply converting the reception status of each MPDU in the receivedPSDU into a Block Ack Bitmap. A Compressed Block Ack differs from theBlock Ack defined in IEEE 802.11e in that there is no need to generate aBlock Ack frame by searching a reception status cache on the basis ofthe Block Ack Starting Sequence Control value indicated by a Block AckRequest.

Assume that, as shown in FIG. 7, of the plurality of MPDUs aggregatedinto the PSDU of the physical frame received by the receiving terminal,the MPDUs with sequence numbers “2”, “5”, and “7” have been found to beincorrect as a result of FCS calculation. In this case, since the firstMPDU has been successfully received, the value of the sequence number ofthe first MPDU can be simply set as the Block Ack Starting SequenceControl value of a Block Ack Request. The receiving terminal returns aCompressed Block Ack having a Compressed Block Ack Bitmap 70 written as10110101 . . . with the Block Ack Starting Sequence Control value beingset to “1”. Upon receiving this Compressed Block Ack, the datatransmitting terminal knows from the value of the Compressed Block AckBitmap 70 that the MPDUs with sequence numbers “1”, “3”, “4”, “6”, and“8” have been successfully transmitted, and determines that the MPDUswith other sequence numbers, i.e., “2”, “5”, and “7”, need to beretransmitted.

Subsequently, as shown in FIG. 8, the transmitting terminal generates aphysical frame having both an MPDU 80 to be retransmitted and an MPDU 81with a new sequence number aggregated in the same PSDU, and transmitsthe frame. Note that how many MPDUs 81 to be newly transmitted areaggregated in the PSDU depends on the buffer size on the receiving side.In the first embodiment, each sequence number is premised on a fieldsize of 12 bits as in the IEEE 802.11 standard, by which numbers from 0to 4095 can be expressed. The sequence number increases one by one from0, and returns to 0 after 4095. Assume that, as shown in FIG. 8, thereceiving terminal has already stored the MPDU with sequence number “0”in the reception buffer 80 before receiving the MPDUs with sequencenumbers “1” to “8”. In addition, the MPDU with sequence number “0”cannot be released from the reception buffer 80 until the MPDU withsequence number “4095” is received.

According to the first embodiment of the present invention, thereceiving terminal manages the reception buffer 80 in the followingmanner. Assume that when a physical frame in which a plurality of MPDUsare aggregated into a PSDU is received, an FCS calculation resultindicates that the first MPDU in the PSDU has been successfullyreceived. In this case, the receiving terminal manages the receptionbuffer 80 by using the sequence number information of this MPDU. In thiscase, managing the reception buffer 80 includes determining an MPDU, ofthe MPDUs which have already been stored in the reception buffer 80,which can be released, and extracting it from the reception buffer 80.The reception buffer 80 is corresponding to reception buffer 113 in FIG.1.

As shown in FIG. 8, for example, upon receiving a physical frame inwhich the MPDUs with sequence numbers “1” to “8” are aggregated, thereceiving terminal determines whether or not the FCS of the first MPDU(corresponding to the MPDU with sequence number “1”) is correct. If theFCS of the first MPDU is correct, the receiving terminal sets thesequence number of this MPDU as a formal Block Ack Starting SequenceControl value. The receiving terminal then determines that MPDUs of theMPDUs stored in the reception buffer 80 which are older than thesequence number “1” can be released from the reception buffer 80. Thereceiving terminal extracts these MPDUs from the reception buffer 80 andforwards them to the upper layer. In this case, the upper layer is anupper protocol layer relative to the MAC layer 102 shown in FIG. 1. Inthis case, MPDUs 82 with sequence numbers “0” and “1” are forwarded to alink layer 103 (more specifically, LLC (Logical Link Control)).According to the first embodiment, since the terminal can extract theMPDU with sequence number “0” from the reception buffer 80 withoutwaiting for the reception of the MPDU with sequence number “4095”, thereception buffer efficiency, i.e., the MAC efficiency, can be improved.Referring to FIG. 8, since the Block Ack Starting Sequence Control valuefrom transmitting terminal is recognized as “1”, the frame of sequencenumber “0” preceding “1” in the reception buffer 80 is releasedtherefrom and forwarded to the upper layer. After all the contents ofthe buffer are released, the frame with sequence number “1” is regardedas a frame consecutively received, and is released from the receptionbuffer 80.

FIGS. 9 and 10 show another example of reception buffer management. Asshown in FIG. 9, the terminal which has received a physical frame havingMPDUs with sequence numbers “1” to “8” aggregated in a PSDU determinesas a result of FCS calculation that the MPDUs with sequence numbers “3”,“4”, “6”, and “8” have been successfully received. Assume that theterminal has already stored the MPDU with sequence number “0” whilewaiting for the reception of the MPDU with sequence number “4095”. Inthis case, since the first aggregated MPDU is incorrect, informationindicating up to which MPDU has been consecutively and successfullyreceived in the receiving terminal is not updated. For this reason, theMPDUs stored in the reception buffer 80 cannot be forwarded to the upperlayer at this point of time.

According to the first embodiment of the present invention, the value ofthe sequence number of the first MPDU, in the PSDU in which a pluralityof MPDUs are aggregated, which has successfully received is written in aBlock Ack Starting Sequence Control 91 of a Compressed Block Ack 90. Inthe example shown in FIG. 9, the MPDU with sequence number “3” is thefirst MPDU which has been successfully received. A Compressed Block AckBitmap 92 is generated with sequence number “3” being set as a startingpoint. In the example shown in FIG. 9, since the first aggregated MPDUis incorrect, although the terminal returns the Compressed Block Ack 90,the terminal releases no MPDU from the reception buffer 80. So, MPDUswith sequence numbers “0”, “3”, “4”, “6” and “8” are stored in thereception buffer.

Upon receiving the Compressed Block Ack 90, the data transmittingterminal regards all MPDUs with sequence numbers before the value of theBlock Ack Starting Sequence Control 91 as retransmission targets, asshown in FIG. 10. That is, in the example shown in FIGS. 9 and 10, theMPDUs with sequence numbers “1” and “2” are regarded as retransmissiontargets. The data transmitting terminal further determines from theCompressed Block Ack Bitmap 92 that the MPDUs with sequence numbers “5”and “7” need to be retransmitted, and transmits them upon aggregating atotal of four MPDUs. At this time, “1” is set in the retransmissionidentifier (Retry bit filed) of the MAC header of each MPDU.

When the receiving terminal determines as a result of FCS calculationthat the first MPDU of the MPDUs aggregated in the PSDU of the physicalframe has been successfully received, buffer management is executed inthe above manner to update the information for buffer management. TheMPDU with sequence number “0” before the Block Ack Starting SequenceControl value “1” is unconditionally forwarded to the upper layer evenin a situation in which MPDUs are discontinuously received in thereception buffer 80. In addition, since the MPDUs with sequence numbers“1”, “2”, and “5” have been successfully received as shown in FIG. 10,all the MPDUs with consecutive sequence numbers “1” to “6” can bereleased from the reception buffer 80 and forwarded to the upper layer.That is, seven MPDUs 110 with consecutive sequence numbers “0” to “6”are released from the reception buffer 80 and forwarded to the upperlayer. In addition to the method of causing the data transmitting sideto determine, on the basis of the Block Ack Starting Sequence Controlvalue of a Compressed Block Ack, which MPDUs the terminal has failed totransmit, and transmitting data to be retransmitted upon aggregatingthem, a method of notifying a destination terminal of a proper Block AckStarting Sequence Control value by transmitting only a Block Ack Requestcan also be applied to the present invention. In the case, in theexample shown in FIGS. 9 and 10, the transmitting terminal transmits aBlock Ack Request with the Block Ack Starting Sequence Control valuebeing designated to “1”, and the data receiving terminal returns aCompressed Block Ack with sequence number “1” as a starting point.

Consider a case wherein, in the example shown in FIGS. 7 and 8, the datatransmitting side has not been successfully notified of the CompressedBlock Ack transmitted by the data receiving terminal due to an error. Inthe case shown in FIG. 8, the Compressed Block Ack with the Block AckStarting Sequence Control value “1” is transmitted. If an error occursin this Compressed Block Ack, the transmitting side causes a timeoutafter a lapse of a predetermined period of time, and transmits a BlockAck Request with the Block Ack Starting Sequence Control value “1”. Thereceiving side returns a Compressed Block Ack based on the Block AckStarting Sequence Control value of the Block Ack Request. In this case,the receiving side can release a frame from the reception buffer 80 andforward it to the upper layer even if the receiving side has notreceived the retransmitted Block Ack Request. This is because, when aPSDU in which a plurality of MPDUs are aggregated is received, as longas the FCS for the first MPDU is correct, reception buffer managementcan be done by regarding the sequence number of the first MPDU as aBlock Ack Starting Sequence Control value. When a Block Ack Request isretransmitted with the Block Ack Starting Sequence Control value beingset to “1”, the frames with sequence numbers “3” and “4” stored in thereception buffer 80 in FIG. 8 are not released from the buffer and arekept stored therein because their sequence numbers are newer than theBlock Ack Starting Sequence Control value.

In the example shown in FIG. 7, if all the aggregated MPDUs areincorrect, the data receiving side can neither update the Block Ackstarting sequence control value nor return any Compressed Block Ack. Forthis reason, the frame with sequence number “0” which has already beenstored in the reception buffer 80 is not released from the buffereither. The data transmitting side detects a timeout after a lapse of apredetermined period of time, and transmits a Block Ack Request as aBlock Ack Starting Sequence Control value. The data receiving side hasnot received any MPDU after sequence number “1”, and hence returns theBlock Ack Bitmap with all values being set to 0 as the Block AckStarting Sequence Control value “1”. At this time, the frame withsequence number “0” stored in the reception buffer 80 is released fromthe buffer and forwarded to the upper layer because the sequence numberis before the Block Ack Starting Sequence Control value.

A buffer management technique based on the consideration of the windowsize on the receiving side will be described with reference to FIG. 11.The window size indicates the maximum number of MPDUs which can bestored on the receiving side. In the example shown in FIG. 11, a maximumof eight MPDUs can be stored, and hence the window size is 8. Assumethat MPDUs 111 with sequence numbers “4095” and “0” have already beenstored in the reception buffer 80, as shown in FIG. 11. Assume that thetransmitting terminal transmits new MPDUs with sequence numbers “1” to“8” upon aborting retransmission, and all the MPDUs are successfullyreceived. In this case, the reception buffer 80 overflows.

According to the first embodiment of the present invention, therefore,in order to prevent the reception buffer 80 from overflowing, MPDUswhich are not allowed to stay in the reception buffer 80 are determinedon the basis of the sequence number of the last MPDU which has beensuccessfully received and the value of the window size, and all thedetermined MPDUs are extracted from the reception buffer 80 andforwarded to the upper layer. More specifically, all the frames havinglower sequence numbers than the sequence number preceding the last MPDUby the window size are extracted from the reception buffer 80 andforwarded to the upper layer. In the example shown in FIG. 11, since theMPDU with sequence number “8” is the last MPDU which has beensuccessfully received, the MPDUs 111 with sequence numbers “4095” and“0” are unconditionally released from the reception buffer 80. As aconsequence, only the MPDUs with sequence numbers “3”, “4”, “6”, and “8”are stored in the reception buffer 80.

When a plurality of MPDUs are to be transmitted at once to a pluralityof destinations by using the format in which the length information ofthe aggregated MPDUs is written in the head of the PSDU as shown in FIG.4, each receiving terminal can regard the sequence number of the firstMPDU as a Block Ack Starting Sequence Control value in a state whereinthe terminal can recognize from which part of the PSDU the MPDUaddressed to itself starts. That is, the first embodiment of the presentinvention can be applied not only to data transmission to a signaldestination but also to data transmission to a plurality ofdestinations.

As described above, according to the first embodiment of the presentinvention, the reception buffer 80 can be managed in the above mannerupon successful reception of the first MPDU of MPDUs aggregated in aPSDU. This makes it possible to prevent any MPDUs from undesirablystaying in the reception buffer 80 and improve the reception bufferefficiency, i.e., the MAC efficiency.

Second Embodiment

The second embodiment of the present invention is directed to acommunication apparatus which makes a receiving terminal clear areception buffer by transmitting only a Block Ack Request when atransmitting terminal determines that the transmission schedulingintervals between QoS data frames or QoS CF-Poll frames are relativelylong.

HCCA is associated with an extended scheme of conventional PCF (PointCoordination Function). More concretely, the HC (Hybrid Coordinator)performs bandwidth management including the allocation of TXOPs to QSTA(QoS stations). The HC is collocated with a QAP. At the start ofcommunication, a QSTA sets up a TS (Traffic Stream). The TS is a set ofMSDUs to be delivered subject to the QoS parameter values provided tothe MAC in a particular TSPEC (Traffic Specification). TSPEC is the QoScharacteristics of a data flow to and from a non-AP QSTA. TSs are onlymeaningful to MAC entities that support QoS within the MAC data service.When the setup of a TS is started, a TSPEC is notified from the QSTA.The TSPEC contains information such as a TSID (Traffic StreamIdentifier) and Mean Data Rate (that specifies the average data ratespecified at the MAC-SAP, in units of bits per second.) A plurality ofTSs can be set. Each HC must perform scheduling so as to satisfy TSrequirements. More specifically, when performing scheduling, the HCdetermines a transmission cycle for data to a certain destination andsends QoS CF-Poll on the basis of service interval information called“Service Interval”. The QSTA obtains a TXOP period by polling from theHC. This allows the QSTA to transmit a frame during this period.

The second embodiment of the present invention will be described withreference to FIGS. 12 and 13. Referring to FIG. 12, in a contention-freeHCCA period, a TXOP (transmission opportunity) from an HC to a QSTA isset as a TXOP period A, and TXOP periods for transmission from the HC toother QSTAs are set as TXOP periods B to Z. At the end of the HCCAperiod, a CF-End (Contention Free End) frame 300 indicating the start ofa contention period is broadcast. Referring to FIG. 12, assume that theHC has transmitted a plurality of MPDUs with consecutive sequencenumbers “1” to “4” upon aggregating them, and the FCS calculation resultindicates that the MPDU with sequence number “1” is incorrect. Assumethat a reception buffer 301 of the QSTA in FIGS. 12 and 13 has a sizethat allows a maximum of five MPDUs to be stored, and the frame withsequence number “0” has already been stored in the reception buffer 301of the QSTA. In this case, if the TXOP periods for the remaining QSTAsprolong, the Lifetime of a MAC frame may be exceeded. According to theIEEE 802.11e/Draft 10.0 standard, a Delay Bound is set in accordancewith the priority of traffic. Any MAC frame that has exceeded the DelayBound is discarded upon aborting transmission even if the frame needs tobe retransmitted. Referring to FIG. 12, when the TXOP periods for theremaining QSTAs prolong and the MPDU with sequence number “1” whichneeds to be retransmitted is discarded, the QSTA cannot release theMPDUs with sequence numbers “0”, “2”, “3”, and “4” stored in thereception buffer 301 from the buffer and forward them to the upperlayer. The reception buffer 301 of the QSTA overflows unless the datatransmitting side uses the method like the first embodiment whichabandons retransmitting the MPDU with sequence number “1” which hasexceeded the Delay Bound, transmitting the MPDUs with sequence numbers“5” to “8”, and regarding the sequence number “5” as a Block AckStarting Sequence Control value. Therefore, the MPDUs with sequencenumbers “5” to “8” must be discarded before an MPDU is stored in thereception buffer 301. For this reason, as shown in FIG. 13, in thesecond embodiment of the present invention, if the transmitting terminalsends a block of QoS data frames for a predetermined period, a Block AckRequest is transmitted to clear the reception buffer 301. Referring toFIG. 13, in order to release frames up to sequence number “4” from thereception buffer 301, a Block Ack Request 302 with a Block Ack StartingSequence Control value being set to “5” is transmitted at the start ofthe second TXOP period A (TXOP for data transmission to the QSTA).According to the IEEE 802.11e/Draft 10.0 standard, since all MAC frameshaving sequence numbers lower than the Block Ack Starting SequenceControl value need to be released from the reception buffer andforwarded to the upper layer, the frames with sequence numbers “0”, “2”,“3”, and “4” are released from the reception buffer 301. Even if theMPDUs with sequence numbers “5” to “8” are transmitted afterward, thereis no chance that the buffer will flow.

Another example of the second embodiment of the present invention willbe described with reference to FIGS. 14 to 16. Assume that, as shown inFIG. 14, the HC transmits MPDUs with sequence numbers “1” to “4” and aBlock Ack Request with an Block Ack Starting Sequence Control valuebeing set to “1” are transmitted upon aggregating them into one PSDU inthe TXOP period A. Assume that one of the aggregation methods shown inFIGS. 4 and 5 in the first embodiment described above is used. In thiscase, if the FCS of the MPDU with sequence number “1” is incorrect, theMPDUs with sequence numbers “2” to “4” are stored in the buffer on thereceiving side. A cycle in which data is transmitted to a QSTA 1 isbased on the SI (Service Interval) of the QSTA 1. The SI is the intervalbetween the start of two successive scheduled SP (Service Periods). A SPis a contiguous time during which one or more downlink unicast framesare transmitted to a QSTA and/or one or more TXOPs are granted to thesame QSTA. SPs can be scheduled or unscheduled. The scheduled serviceperiod is scheduled by the QAP, and starts at fixed intervals of time.As the service start timing for a QSTA 2 approaches, data transmissionto the QSTA 2 must be started while transmission to the QSTA 1 isinterrupted for a while. If this state continues, the MPDUs withsequence numbers “2” to “4” stored in the reception buffer of the QSTA 2will be wasted.

In the second embodiment of the present invention, therefore, if theremainder of the TXOP period (channel use permission period) is tooshort to retransmit the incorrect MPDUs, only a Block Ack Request istransmitted to the destination terminal to clear the reception buffer ofthe destination terminal and forward all the MPDUs to the upper layer.

In the example shown in FIG. 14, the Block Ack Starting Sequence Controlvalue of a Block Ack Request 120 to be transmitted just before the endof a TXOP period A is “5”, and when the HC transmits the Block AckRequest 120, the QSTA 1 which is the destination terminal assigned withthe TXOP period A forwards all the MPDUs with sequence numbers “2” to“4” to the upper layer.

Operation in a TXOP period B following the TXOP period A is similar tothat described above. In the TXOP period B, the HC transmits MPDUs withsequence numbers “1001” to “1003” and a Block Ack Request with a BlockAck Starting Sequence Control value being set to “1001” to the QSTA 2upon aggregating them. The QSTA 2 is a destination terminal differentfrom the QSTA 1. If it is determined as a result of FCS calculation thatthe MPDU with sequence number “1001” is incorrect, the MPDUs withsequence numbers “1002” and “1003” are stored in the reception buffer inthe QSTA 2 and cannot be forwarded to the upper layer. When a Block Ack121 is returned to the HC, the HC determines that the MPDU with sequencenumber “1001” needs to be retransmitted. However, since the remainder ofa predetermined contention-free HCCA period is small, and an EDCA periodduring which contention is performed approaches, the HC cannot afford toperform retransmission processing.

In general, in an HCCA period, traffic with high priorities whichrequire scheduling is transmitted, and in an EDCA period (contentionperiod), best-effort type traffic with relatively low priority istransmitted. When the HCCA period switches to the EDCA period,therefore, the HC may transmit data with different priorities which areaddressed to a destination different from the QSTA 2 upon aggregatingthem.

In the second embodiment, when the EDCA period starts, the HC transmits,to the QSTA 2, a Block Ack Request 122 to clear the reception buffer,with the highest priority. In EDCA, a plurality of ACs (AccessCategories) are provided for the respective priorities, whichconcurrently perform CSMA/CA in parallel. Each AC has its own IFS (AIFS)period. ACs with higher priorities enable the start of transmission of aframe by carrier sense for shorter periods of time. When transmissionprocessing simultaneously occurs between a plurality ACs within MAClayer, a frame in an AC with a high priority is transmitted, and an ACwith a low priority is set in a random backoff wait time again uponincreasing the CW (Contention Window). In the example shown in FIG. 14,immediately after the start of the EDCA period, the Block Ack Request122 to make the QSTA 2 clear the reception buffer is set in the head ofa transmission queue in the AC with the highest priority. This makes itpossible to transmit the Block Ack Request 122 to the QSTA 2 before theHC transmits data to another terminal (and/or another priority) in theEDCA period. The MPDUs with sequence numbers “1002” and “1003” stored inthe reception buffer of the QSTA 2 are forwarded to the upper layerbefore the arrival of the MPDU with sequence number “1001”. In thiscase, the transmitting side preferably designates an appropriate valueas the Block Ack Starting Sequence Control value of the Block AckRequest 122 to clear the reception buffer. If, for example, it isdetermined that the delay limits of MPDUs (which are decided withrespect to each application) stored in the buffer on the receiving sidewill be exceeded, they are forwarded to the upper layer even when thesequence numbers are discontinuous.

FIG. 15 shows an example of how the present invention is applied to acase wherein a plurality of MPDUs are transmitted after being aggregatedwithout containing any Block Ack Request. This procedure will bereferred to as a “Implicit Block Ack Request” hereinafter. The HCtransmits an explicit Block Ack Request 130 to the QSTA 1 to make itclear the reception buffer. Likewise, the HC transmits an explicit BlockAck Request 131 to the QSTA 2. FIG. 16 shows an example of how thepresent invention is applied to the Block Ack technique defined in IEEE802.11e. The HC forwards a Block Ack Request 140 to the QSTA 1 to makeit clear the reception buffer. Likewise, the HC transmits an explicitBlock Ack Request 141 to the QSTA 2.

According to the second embodiment of the present invention, if theremainder of an HCCA period is small, and retransmission processingcannot be performed because an EDCA period approaches, an HC transmits aBlock Ack Request to clear the reception buffer. This makes it possibleto prevent MPDUs from undesirably staying in the reception buffer andimprove the reception buffer efficiency, i.e., the MAC efficiency.Obviously, the second embodiment of the present invention can be appliednot only to downlink trans-mission from an HC to a QSTA but also touplink transmission from a QSTA to an HC.

The second embodiment of the present invention can also be applied todata transmission in an EDCA period in the contention-based QoS accesscontrol scheme. When EDCA is used, strict schedule management as in HCCAcannot be done. For this reason, a terminal which transmits data checksthe contents of a transmission queue for each AC. Upon determining thatan opportunity to transmit data to the current destination will not beavailable for a while, the terminal transmits a Block Ack Request torelease the reception buffer at the end of the TXOP period, therebyproperly performing buffer management. MAC frames are extracted from atransmission queue according to the FIFO (First In First Out) principleby searching the queue from its head. For this reason, as a frame to thedestination is stored closer to the end of the queue, the frame istransmitted at a later time. The second embodiment may also use a methodof transmitting a Block Ack Request to release the reception buffer atthe start of a TXOP period if a predetermined period of time has elapsedat the start of a new EDCA period since the end of the preceding EDCAperiod. According to IEEE 802.11-Standard 1999, the More Data field ofthe MAC header with 1 bit length is used to indicate to a STA inpower-save mode that more MSDUs, MMPDUs (management MPDUs) are bufferedfor that STA at the AP. This field is valid in directed data ormanagement type frames transmitted by an AP to a STA. If there issucceeding data or a succeeding management frame with respect to theterminal (station) from the viewpoint of an access point, this flag isset to 1. If no succeeding frame is present, the flag is set to 0. Ifthe More Data flag is set to 0, the terminal which has received theframe shifts to the power save mode. The data frame receiving terminalmay manage the reception buffer according to the Block acknowledgementprocedures defined in IEEE 802.11e/Draft 10.0 while this more data flagis set to 1. The terminal may also release frames from the receptionbuffer and forward them to the upper layer, when the flag is set to 0,upon determining that no data addressed to itself will be transmittedfor a while. This method can be applied to data transmission in both anEDCA period and an HCCA period. The second embodiment of the presentinvention can prevent MPDUs from undesirably staying in the receptionbuffer, thereby improving the reception buffer efficiency, i.e., the MACefficiency.

Third Embodiment

The third embodiment of the present invention is directed to a method ofcreating a Block Ack Bitmap contained in a Block Ack. The maximum lengthof the Block Ack Bitmap in the Compressed Block Ack described in thefirst embodiment is 64 bits. However, the size of the Compressed BlockAck can be further reduced by matching the Compressed Block Ack Bitmapto the size of the reception buffer. In this case, transmitting terminaland receiving terminal should negotiate the information about the sizeof Compressed Block Ack Bitmap corresponds to the size of receptionbuffer. Assume that the size of the reception buffer is set tocorrespond to eight MPDUs. In this case, a maximum of eight MPDUs can beaggregated in a single PSDU in a physical frame to be transmitted by atransmitting terminal, and hence the Compressed Block Ack Bitmap size iseight bits.

Assume that, as shown in FIG. 17, a transmitting terminal transmitsMPDUs with consecutive sequence numbers “1” to “8”, and errors in theMPDUs with sequence numbers “1”, “2”, “5”, and “7” are detected by anFCS check. When the receiving terminal creates a Compressed Block AckBitmap 150 for a Compressed Block Ack, the Compressed Block Ack Bitmap150 is expressed as “11010100” with the Block Ack Starting SequenceControl value “3” being a starting point. As described above, the BlockAck Starting Sequence Control value of a Compressed Block Ack is equalto the sequence number of the first MPDU of the MPDUs aggregated in aPSDU, which has been properly received. Upon receiving the CompressedBlock Ack having the Compressed Block Ack Bitmap 150, the transmittingterminal transmits (retransmits) a physical frame in which the MPDUswith sequence numbers “1”, “2”, “59”, and “7” whose transmission hasfailed are aggregated in one PSDU. In this case, the sequence numbers ofthe MPDUs aggregated in the PSDU are discontinuous. Assume that evenafter this retransmission, the MPDU with sequence number “7” isincorrect on the receiving terminal side, and a Block Ack Bitmap is tobe created from the relative positional relationship between the MPDUs.In this case, the value of Block Ack Starting Sequence Control 151 of aCompressed Block Ack is set to “1”, and a Compressed Block Ack Bitmap152 is created as “11100000” with “1”s being packed from the head. Andthe size of Compressed Block Ack Bitmap can be reduced to 8 bits.

FIG. 18 shows how frames are exchanged when a Compressed Block AckBitmap is to be created from the relative positional relationshipbetween MPDUs. The data transmitting side transmits a physical frame 160in which a plurality of MPDUs are aggregated into one PSDU. Assume thatno Block Ack Request is contained in the physical frame 160 (ImplicitBlock Ack Request). Assume that a result of FCS calculation in thereceiving terminal indicates the MPDUs with sequence numbers “1” and “3”are incorrect. Since the sequence number of the first MPDU which hasbeen successfully received is “2”, the receiving terminal sets the BlockAck Starting Sequence Control value to “2”, and creates a CompressedBlock Ack Bitmap from the relative positional relationship between MPDUsin the aggregated PSDU with “2” being a starting point, thustransmitting a Compressed Block Ack 161. In the example shown in FIG.18, the Compressed Block Ack Bitmap is set as “10100000”. And the sizeof Compressed Block Ack Bitmap is 8 bits which corresponds to the sizeof reception buffer. Upon receiving the Compressed Block Ack 161, thedata transmitting terminal performs CSMA/CA procedure, and thentransmits a physical frame 162 upon aggregating the MPDUs with sequencenumbers “1” and “3” which need to be retransmitted. The example shown inFIG. 18 is premised on the contention-based access control scheme.Obviously, however, the present invention can be applied to acontention-free access control scheme. If the MPDU with sequence number“1” is determined incorrect on the receiving side afterward, thereceiving terminal transmits a Compressed Block Ack 163 upon setting theBlock Ack Starting Sequence Control value to “3” and the Block AckBitmap as “10000000”. In this case, if an error has occurred in theCompressed Block Ack 163 on the transmission channel, the datatransmitting side performs CSMA/CA procedure and then transmits a BlockAck Request 164 upon setting the Block Ack Starting Sequence Controlvalue to “1”. Since the Block Ack Starting Sequence Control value issmaller than the sequence number of the newest MPDU which has beenreceived by the receiving terminal, i.e., “4” in the example shown inFIG. 18, the receiving terminal automatically returns an Ack (CompressedBlock Ack 165) having the same contents as those of the previouslytransmitted Compressed Block Ack (163 in the example shown in FIG. 18).The Block Ack Starting Sequence Control value of the Compressed BlockAck 165 is “3”, and the Block Ack Bitmap is “10000000”. The datatransmitting terminal then determines that the MPDU with sequence number“1” is incorrect, and retransmits a physical frame 166 containing theMPDU. The transmitting terminal then receives a Compressed Block Ack167. When a Compressed Block Ack is to be returned, the receivingterminal reflexively returns the reception status of a maximum of onepast PSDU without searching a reception status as in the Blockacknowledgement technique defined in IEEE 802.11e. The transmitting sidedetermines MPDUs to be retransmitted on the basis of the Block AckStarting Sequence Control value of the Compressed Block Ack.

FIG. 19 shows another example of creating a Compressed Block Ack Bitmapfrom the relative positional relationship between MPDUs. As in theexample shown in FIG. 18, the data transmitting terminal transmits aphysical frame 170 in which MPDUs with sequence numbers “1” to “4” areaggregated, receives a Compressed Block Ack 171, and retransmits aphysical frame 172 in which the MPDUs with sequence numbers “1” and “3”are aggregated. If the physical frame 172 in which the MPDUs withsequence numbers “1” and “3” are aggregated is destroyed by collision orthe like, the receiving terminal cannot create a new Compressed BlockAck Bitmap because it has received no MPDU. In the transmittingterminal, therefore, the wait period for a Compressed Block Ack causes atimeout, and the transmitting terminal transmits a Block Ack Request 173with the Block Ack Starting Sequence Control value being set to “1”.Upon receiving the Block Ack Request 173, the receiving terminalreflexively returns a Compressed Block Ack with the same contents asthose of the previously transmitted Compressed Block Ack with “2”. Inthis example, since the Block Ack Starting Sequence Control value is“2”, the receiving terminal transmits a Compressed Block Ack 174 inwhich a Block Ack Bitmap is set as “10100000” from the relativepositional relationship between MPDUs with “2” being a starting point.

In this case, since all MPDUs with sequence numbers before the Block AckStarting Sequence Control value of the received Compressed Block Ack 174are considered to have failed, the transmitting terminal can determinethat the MPDU with sequence number “1” is a retransmission target.However, since the Compressed Block Ack Bitmap is created from therelative positional relationship between MPDUs in the aggregated PSDU,the MPDU with sequence number “3” which is equal to or larger than theBlock Ack Starting Sequence Control value is erroneously decoded by thetransmitting terminal as one that has been correctly transmitted fromthe information “10100000” of the Block Ack Bitmap. In this case, theCompressed Block Ack Bitmap is created from the relative position thefirst bit “1” corresponds to the MPDU with sequence number “3”. As aresult, although the MPDU with sequence number “1” is retransmitted atthe next transmission timing, the MPDU with sequence number “3” is notcontained in the frame (physical frame 175).

In the third embodiment of the present invention, the problem that anMPDU to be retransmitted is improperly excluded from retransmissiontargets is handled by one of three kinds of methods to be describedbelow.

The first method will be described first with reference to FIG. 20. Aterminal which has transmitted a plurality of MPDUs upon aggregatingthem stores each sequence number as information.

First of all, the transmitting terminal transmits a physical frame 180in which MPDUs with consecutive sequence numbers “1” to “4” areaggregated, and then stores all sequence numbers information “1”, “2”,“3”, and “4”. Thereafter, the transmitting terminal receives aCompressed Block Ack 181 with the Block Ack Starting Sequence Controlvalue being “2”. In this case, the transmitting terminal determineswhether or not the Block Ack Starting Sequence Control value can be usedto determine frames to be retransmitted. In this case, since the BlockAck Starting Sequence Control value “2” of the received Compressed BlockAck 181 coincides with stored sequence number “2”, this value is used.In other words, frames to be retransmitted are determined on the basisof the Compressed Block Ack 181. With this operation, the transmittingterminal transmits the MPDUs with sequence numbers “1” and “3” asretransmission targets upon aggregating them. The transmitting terminalalso updates the stored sequence number information to “1” and “3”. If aphysical frame 182 in which these MPDUs are aggregated is incorrect as awhole, the transmitting terminal receives a Compressed Block Ack 184with the Block Ack Starting Sequence Control value being “2” aftertransmitting a Block Ack Request 183. In this case, sequence number “2”coincides with neither of sequence number “1” and “3” of the MPDUsaggregated by the data transmitting terminal. For this reason, thetransmitting terminal determines that the Compressed Block Ack 184 isidentical to the Compressed Block Ack 181 received in the past, i.e.,the retransmitted Compressed Block Ack 181, and determines that errorshave occurred in all the MPDUs (the MPDUs with sequence numbers “1” and“3” in this case) transmitted by the transmitting terminal. Moreproperly, the transmitting terminal searches the sequence numberinformation “2” in the stored information. That is, unlike in the aboveexample, frames to be retransmitted are not determined on the basis ofthe Compressed Block Ack 181. Instead, the transmitting terminaltransmits the MPDUs with sequence numbers “1” and “3” again uponaggregating them, and stores “1” and “3” as sequence number information.In this case, if the transmitting terminal receives a Compressed BlockAck 186 with the Block Ack Starting Sequence Control value being “1”,the terminal recognizes the information as correct information, andknows that the transmission has succeeded.

According to the second method, serial numbers are assigned toCompressed Block Acks to prevent past Block Ack frame information frombeing erroneously handled. According to IEEE 802.11e/Draft 12.0, a16-bit field called a Block Ack control field exists in a Block AckRequest frame and Block Ack frame. This field includes a 4-bit TIDfield. The remaining portion is a reserved portion. As shown in FIG.21A, therefore, a 4-bit Block Ack ID (acknowledgement identifier) field190 is newly defined.

As shown in FIG. 21B, every time a physical frame in which a pluralityof MPDUs are aggregated and a Compressed Block Ack Bitmap is newlycreated, the Block Ack ID field 190 is updated. Note that the size ofthe Block Ack ID field 190 is set to be four bits. Obviously, however,the Block Ack ID field 190 is not limited to this size.

FIG. 22 shows an example of how frames are exchanged when a Block Ack IDfield is used. First of all, assume that when the receiving terminalreceives a physical frame 200 in which MPDUs with sequence numbers “1”to “4” are aggregated and returns a Compressed Block Ack 201, theterminal sets the Block Ack ID to 0. The data transmitting side receivesthe Compressed Block Ack 201 and holds the value of the Block Ack ID inthe Compressed Block Ack 201. Assume that the MPDUs with sequencenumbers “1” and “3” are aggregated as MPDUs to be retransmitted, but aphysical frame 202 is destroyed as a whole. When the data transmittingterminal transmits a Block Ack Request 203 and then receives aCompressed Block Ack 204 with the Block Ack ID being 0, the terminal canrecognize that the frame of the Compressed Block Ack 204 has the samecontents as those of a frame received in the past, and regards that thetransmission of all the MPDUs (i.e., all the MPDUs aggregated in thephysical frame 202) has failed.

Subsequently, the MPDUs with sequence numbers “1” and “3” areretransmitted (physical frame 205). When newly creating a CompressedBlock Ack 206, the receiving terminal updates the value of the Block AckID to 1. Since the Block Ack ID is updated, the transmitting terminaldetermines that the information of the Compressed Block Ack 206 iscorrect. In the example shown in FIG. 21A, since the Block Ack ID isfour bits, this value returns to 0 after 7.

In the third method, upon receiving a Compressed Block Ack, a datatransmitting terminal stores all the information or a copy of the frameitself. The application of the third method is conceptually the same asthat of the first method. According to this method, it is determined bythe collation of the bit string of a Compressed Block Ack whether or notit was received in the past.

In addition to the method of creating a Compressed Block Ack Bitmap fromthe relative positional relationship between MPDUs, a method of creatinga Compressed Block Ack Bitmap in correspondence with sequence numbers isavailable. This method will be described with reference to FIG. 23.Assume that, as shown in FIG. 23, the data transmitting terminaltransmits MPDUs with sequence numbers “1” to 11811 upon aggregatingthem. If the FCS calculation result obtained by the receiving terminalindicates that the MPDUs with sequence numbers “1”, “2”, “5”, and “7”are incorrect, a Block Ack Starting Sequence Control 210 is set to11311, and a Block Ack Bitmap 211 is set as “11010100000 . . . ”. Inthis case, the Block Ack Bitmap 211 has a fixed length of 64 bits.According to IEEE 802.11e/Draft 10.0, the maximum number of MSDUs whichcan be consecutively transmitted by the Block Ack technique is 64, andthe bitmap can be compressed to 64 bits as in the first embodimentdescribed above on the premise that no fragmentation of MSDU isperformed. Thereafter, the data transmitting terminal transmits theMPDUs sequence numbers “1”, “2”, “5”, and “71” upon aggregating them onthe basis of the contents of the Compressed Block Ack.

The receiving side then determines that the MPDU with sequence number“7” is incorrect. The value of a Block Ack Starting Sequence Control 212is equal to the sequence number of the first aggregated MPDU, i.e., “1”,and the reception status of each MPDU is converted into a bitmap with“1” being a starting point. That is, since the MPDUs with sequencenumbers “1”, “2”, and “5” have been successfully received, a Block AckBitmap 213 is expressed as “11001000 . . . ”. The physical framereceived by the receiving terminal at this point of time does notcontain the MPDUs with sequence numbers “3” and “4”, and thecorresponding bits of the Compressed Block Ack Bitmap 213 are kept to be0. In addition, since the reception of the MPDU with sequence number “7”has failed, the corresponding bit is kept to be 0. Note that when a bitconfiguration is mutually recognized by both a transmitting terminal anda receiving terminal, negative logic may be used in place of positivelogic.

FIG. 24 shows an example of how frames are exchanged when a CompressedBlock Ack Bitmap is created in correspondence with sequence numberpositions. The data transmitting terminal transmits a physical frame 220in which MPDUs with sequence numbers “1” to “4” are aggregated. Thereceiving side has successfully received the MPDUs with sequence numbers“2” and “4”, and returns a Compressed Block Ack 221 upon setting theBlock Ack Starting Sequence Control value to “2” and the Block AckBitmap to “10100000 . . . ”. Since the MPDUs with sequence numbers “1”and “3” are incorrect, the data transmitting terminal transmits aphysical frame 222 in which these MPDUs to be retransmitted areaggregated. The overall physical frame 222 is destroyed by collision orthe like, and a Block Ack Request 223 with the Block Ack StartingSequence Control value being set to “1” is transmitted. The receivingside returns a Compressed Block Ack 224 with the same contents as thoseof the previously returned Compressed Block Ack. The transmittingterminal regards the MPDU with sequence number “1” before the Block AckStarting Sequence Control value “2” as one that the terminal has failedto transmit. Assume that the Block Ack Starting Sequence Control valueis “2” and the Block Ack Bitmap is set to “10100000 . . . ”. In thiscase, since this information is created on the basis of the sequencenumber positions, the bit information corresponding to sequence number“3” is 0, and it can be determined that the transmission has failed.Therefore, the MPDUs with sequence numbers “1” and “3” to beretransmitted are aggregated to transmit a physical frame 225.

As described above, a Compressed Block Ack Bitmap in a Compressed BlockAck can be created by roughly two types of methods. Informationindicating which one of the methods was used to create a CompressedBlock Ack Bitmap is recognized by the transmitting and receivingterminals through negotiation. Alternatively, the terminals may be madeto recognize such information by using one bit of the reserved field ofthe Block Ack Control field shown in FIG. 21, as needed.

According to the embodiments of the present invention described above,the reception buffer in a receiving terminal can be managed uponsuccessful reception of the first MPDU of the MPDUs aggregated in aPSDU. This prevents MPDUs from undesirably staying in the receptionbuffer and can improve the reception buffer efficiency, i.e., the MACefficiency. If the remainder of an HCCA period is small, andretransmission processing cannot be performed because an EDCA periodapproaches, an HC transmits a Block Ack Request to clear the receptionbuffer. This makes it possible to prevent MPDUs from undesirably stayingin the reception buffer and obtain the same effects as those describedabove. In addition, variably changing the length of a Compressed BlockAck Bitmap in accordance with the number of MAC frames to be aggregatedmakes it possible to solve the problem that when retransmission occurs,the transmitting terminal side erroneously decodes a Compressed BlockAck frame in the past.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1-3. (canceled)
 4. A communication apparatus comprising: a physicalframe transmitting device configured to acquire a channel use permissionperiod, and transmit a physical frame to a destination communicationapparatus in the channel use permission period; and an acknowledgementrequest frame transmitting device configured to transmit anacknowledgement request frame to the destination communicationapparatus, at a start of the channel use permission period, wherein astarting sequence number of the acknowledgement request frame is setlarger than any sequence numbers of transmitted data frames, in orderthat the destination communication apparatus forwards to an upper layera data frame which has already been stored in a reception buffer of thedestination communication apparatus.
 5. An apparatus according to claim4, wherein the acknowledgement request frame transmitting devicedetermines, on the basis of intervals of transmission of physical framesby the physical frame transmitting device, whether or not to transmitthe acknowledgement request frame.
 6. An apparatus according to claim 4,wherein the acknowledgement request frame transmitting device transmitsthe acknowledgement request frame with a highest priority immediatelyafter the start of a contention period following a contention freeperiod.
 7. (canceled)
 8. The apparatus according to claim 4, furthercomprising: a receiving device configured to receive an acknowledgementframe having an acknowledgement identifier in response to thetransmitted physical frame; a storage device to store theacknowledgement identifier of the received acknowledgement frame; adetermination device configured to, when a first acknowledgement frameis received by the receiving device, determine whether or not anacknowledgement identifier of the first acknowledgement frame coincideswith an acknowledgement identifier of a second acknowledgement framewhich has already been received by the receiving device and stored inthe storage device; and a retransmitting device configured to determineMAC frames to be retransmitted, on the basis of a determination resultobtained by the determination device, and retransmit a physical frame inwhich the MAC frames are aggregated.
 9. The apparatus according to claim4, further comprising: a receiving device configured to receive anacknowledgement frame having an acknowledgement bitmap in response tothe transmitted physical frame; a storage device to store theacknowledgement bitmap of the received acknowledgement frame; adetermination device configured to, when a first acknowledgement frameis received by the receiving device, determine whether or not anacknowledgement bitmap of the first acknowledgement frame coincides withan acknowledgement bitmap of a second acknowledgement frame which hasalready been received by the receiving device and stored in the storagedevice; and a retransmitting device configured to determine MAC framesto be retransmitted, on the basis of a determination result obtained bythe determination device, and retransmit a physical frame in which theMAC frames are aggregated. 10-13. (canceled)
 14. A communication methodcomprising: acquiring a channel use permission period, and transmittinga physical frame to a destination communication apparatus in the channeluse permission period; and transmitting an acknowledgement request frameto the destination communication apparatus at a start of the channel usepermission period wherein a starting sequence number of theacknowledgement request frame is set larger than any sequence numbers oftransmitted data frames, in order that the destination communicationapparatus forwards to an upper layer, a data frame which has alreadybeen stored in a reception buffer of the destination communicationapparatus. 15-20. (canceled)